It is known and desireable in the semiconductor arts to contact a semiconductor substrate through the top of a semiconductor structure. One such method of making substrate contact is to diffuse dopant from the top surface of the structure to the substrate. Although contact may be made in this manner, the diffusion likely spreads and consumes valuble space. Further, this method may not be utilized in a structure having a blanket buried layer.
Trench structures are also well known in the semiconductor arts and are employed for making substrate contact as well as for device and circuit isolation. Trenches employed in these manners are especially useful in high performance bipolar and BICMOS integrated circuits.
One method of forming trenches for both isolation and substrate contacts includes first etching an isolation trench, forming a trench liner on the sides and bottom of the trench and then filling the remainder of the trench with undoped polysilicon. Once this isolation trench has been completed, substrate contact trenches are etched and an oxide liner is formed on the sides and bottoms thereof. The liner is then removed from the bottom of the trench exposing the substrate. Once the substrate is exposed, the trench is filled with doped polysilicon to form a substrate contact trench. This process employs a series of masks and requires numerous process steps. This is relatively expensive and requires large amounts of time.
Another method of forming both substrate contact trenches and isolation trenches in the same structure includes simultaneously etching trenches and then filling the trenches with doped polysilicon. Although this is relatively easy to do, the structure suffers from significantly high capacitance and is therefore undesirable.
Another method of forming both substrate contact trenches and isolation trenches in a single structure includes simultaneously etching and then filling the trenches. It is then attempted to selectively dope the trench fill disposed in the substrate contact trenches without doping the trench fill disposed in the isolation trenches. This is very difficult to do and is also expensive and time consuming.
Accordingly, it would be highly desirable to have a method of fabricating a semiconductor structure where substrate contact trenches and isolation trenches could be simultaneously fabricated so that the structure has reduced capacitance, may be employed with a blanket buried layer, is relatively easy to build and is inexpensive.